1. Field of the Invention
The present invention generally relates to an electrically powered fan used to blow air.
2. Description of the Related Art
Conventionally, a centrifugal type fan, taking air in an axial direction and exhausting the air in a radial direction, has the following configuration. Specifically, the conventional fan includes an impeller having a plurality of blades arranged in a circumferential direction centered about a center axis, and a substantially cup-shaped portion arranged at the middle of the impeller into which a substantially cylindrical yoke made of magnetic material is press-fitted. In addition, a field magnet is attached to an inner side surface of the yoke. By virtue of this configuration, the impeller is rotatably supported around the center axis. The blades of the impeller are arranged on radially outer positions of the cup-shaped portion, and the cup-shaped portion and the blades are unitarily formed of synthetic resin, both of which are connected via a joint portion. By virtue of this configuration, a circular space is provided between the plurality of blades and the outer side surface of the cup-shaped portion.
In terms of a centrifugal fan, it may be preferable to enlarge the space provided at an inner side of the plurality of blades (in other words, the space between radially inner end portions of the blades and the outer side surface of the cup-shaped portion, to which the yoke is press-fitted, is made wider). With the wider space, the fan may take more air therein, which results in improved blower efficiency of the fan. However, upon making a diameter of the yoke smaller to enlarge the space, a magnetic circuit will be decreased in size. As a result, the motor efficiency is degraded. Upon making a diameter of the circular space bigger while fixing an outer diameter of the impeller, a blade-area will be decreased in size, which results in degraded blower efficiency. Upon making a diameter of the circular space bigger while keeping the blade-area of the impeller constant, the impeller will be enlarged.
In order to enlarge the circular space without expanding the outer diameter of the impeller or degrading the blower efficiency, it is preferable to omit the cup shaped portion of the impeller covering the outer side surface of the yoke.
In publicly available examples, a portion of the outer side surface around the opening of the permanent-magnet rotor having a cylindrical shape whose top is covered, and an inner side surface of the cylindrical portion provided at a middle of the impeller are fixed by, for example, press-fitting, bonding, and crimp-fixing. In another publicly available example, a flange portion is provided around the outer side surface of the opening of the permanent-magnet rotor, and the flange portion is fixed to the base plate of the centrifugal fan by crimp-fixing.
However, in case that the permanent-magnet rotor and the cylindrical portion arranged at the middle portion of the impeller are press-fitted or bonded, an axial length of an affixing area at which the outer side surface of the permanent-magnet rotor is abutted against the impeller is short. Therefore, the impeller may not be fixed securely to the permanent-magnet rotor by press-fitting or bonding. For crimp-fixing, forming the engaging portion and crimping processes are required, which may deteriorate the work efficiency.
Furthermore, the cup shaped portion of the impeller, which is made of resin, may be broken or cracked by the stress generated upon press-fitting the permanent magnet rotator (i.e., the cylindrical yoke made of metallic material with the field magnet attached to the inner side surface thereof) into the cup-shaped portion. Especially in a large-size fan, it is highly probable that the impeller is damaged or cracked. On the other hand, if the press-fit pressure is reduced, the permanent-magnet rotor may not be securely fixed to the impeller. As a result, the permanent-magnet rotor may detach from the impeller.
In case that such a fan is utilized in a low temperature environment, the impeller made of resin shrinks more than the yoke made of metallic material does, which results in breaking or cracking of the attaching portion of the impeller and the yoke.